Why couldn't Gamefreak make the move Cosmic Power as a TM? , when some TM moves are Calm Mind , Bulk Up and Swords Dance.
It's been a fantasy having an Umbreon that can learn Cosmic Power
That would be quite overpowered. Believe me, you do not want an Espeon with that..holy..
GameFreak simply made it like that. There's not much else to say about it, besides on how much popularity Espy will get :O